Ultraviolet light emitting diodes have widely attracted attention as a next-generation light source to be used for a substitute for fluorescent lamps, a high-density DVD, a laser for biochemistry, decomposition of harmful substances by a photocatalyst, a He—Cd laser, a substitute for mercury-vapor lamps, and the like. Ultraviolet light emitting diodes are composed of an AlGaN nitride semiconductor called a wide gap semiconductor, and laminated on different kinds of substrates, such as sapphire, 4H—SiC, and GaN, shown in Table 1.
However, there is a large lattice mismatch between sapphire and AlGaN, whereby a lot of threading dislocations are present and become the center of non-radiative recombination to remarkably reduce internal quantum efficiency. 4H—SiC and GaN have high lattice matching with AlGaN, but are expensive. In addition, 4H—SiC and GaN absorb ultraviolet rays having a wavelength of not more than 380 nm, and a wavelength of not more than 365 nm, respectively.
Meanwhile, AlN has a lattice constant similar to that of AlGaN and is transparent in an up to 200-nm ultraviolet region, whereby, without absorbing ultraviolet rays emitting light, ultraviolet light can be efficiently taken out to the exterior. In other words, using AlN single crystals as a substrate, an AlGaN light-emitting diode is made to quasihomoepitaxially grow, whereby an ultraviolet light emitting diode which is controlled to have a lower crystal defective density can be produced.
TABLE 1Ultraviolet LEDSubstrateSapphireSiCGaNAlNCostoxxxLattice MatchingxoooUltraviolet LightoΔΔoTransmittance
Currently, using methods, such as an HVPE method (hydride vapor phase epitaxy method), a liquid phase epitaxy method, and a sublimation-recondensation method, trial production of bulk single crystals of AlN has been conducted. For example, PTL 1 discloses that, in a liquid phase epitaxy method of group III nitride crystals, a pressure is applied in order to increase an amount of nitrogen dissolved in a flux, whereby alkali metals, such as sodium, are added to the flux. PTL 2 proposed a method for producing AlN microcrystals by introducing gas containing nitrogen atoms into a melt of Al.
However, when AlN crystals are produced using the technique of PTL 1 or PTL 2, a high growth temperature is needed, and accordingly production costs and crystal quality of AlN crystals are not satisfactory.